4.5.4 Plant hormones (biology only)

4.5.4.1 Control and coordination

Plant Hormones and Movement

Plants require hormones too.

• They are needed for the optimal growth of the plant.
  • For example they are involved in movements towards essential resources for photosynthesis
  • such as water and light.
  • These movements are called tropisms.

Types of Tropism

• Phototropism involves moving in relation to light.
  • This comes from the Greek for light ‘photo’
• Geotropism involves moving in relation to gravity.
  • These are also known as gravitotropism.

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Positive and Negative Tropisms

• Both Phototropisms and Geotropisms have positive and negative forms.
  • Positive phototropism is movement towards the light whilst negative phototropism is movement away from it.

Auxins in the Roots

• In the root, auxins will reduce growth.
  • The auxin will congregate in the shaded side side of the root, however here it reduces growth, so it moves away from the light.

Auxins and Geotropism

• When the stem is horizontal, the auxin will concentrate in the bottom half of the stem,  causing an increase in growth of the bottom part.
• In the root, the auxin will again congregate in the bottom of the root.

Laboratory Experiment

1. Take four coleoptiles

2. Place a lamp to the right of each coleoptile then leave it for a few days

Uses of Auxins

• Auxins have many uses is agriculture and horticulture by harnessing their property in causing the elongation of cells.
• They are used as:
  • Weedkillers
  • Rooting powders
  • To promote growth in tissue culture

Weedkillers

• Weedkillers are used to kill plants that are pests.
  • The weeds compete for resources with the crops, reducing the efficiency of farming.
  • Weedkillers reduce the number of weeds, therefore increasing the crop yield.
  • Selective weedkillers kill only the pests.
  • They contain auxins that make the weeds grow at too quick a rate, killing the plant.

Rooting Powders

• Rooting powders are used to increase the growth rates of plants.
  • They have auxins that are used to increase the growth of plants.
  • Plant stem cuttings are dipped in this powder before being potted.
  • This increases the growth of the roots.

Tissue Culture Growth Promotion

• Auxins are also used in tissue culture.
  • Tissue culture involves growing plants in culture form small cuttings of other plants.
  • The culture is a growth medium in which many growth hormones such as auxins are used.
  • Other nutrients such as glucose and mineral ions will also be present in this culture, to provide the plant with the optimum conditions for growth.

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4.5.4.2 Use of plant hormones (HT only)

Gibberellins

• Gibberellins initiate seed gemination.
  • Plant seeds go through a period called dormancy before they begin germinating.
  • Gibberellins cause this dormancy to end by producing a carbohydrase enzyme called alpha-amylase.

Gibberellins

• Only a low concentration of Gibberellins are required to cause an increase in the speed of germination.
  • They also lead to cell elongation which increases the size of the fruit.
  • Moreover, Gibberellins work to promote the flowering of plants.

Ethene

• Ethene, C2H4, is a hydrocarbon, a substance made from carbon and hydrogen.
  • It can be used to speed up cell division in plants and to ripen the fruit.

Ethene

• This is important in the food industry.
  • As preservatives are used in many foods, a similar principle is important in the fruit industry.
  • Fruits are harvested unripe, then ethene is given.
  • This is used to speed up the ripening process, after fruits have been transported.